The exhaust system for an internal combustion engine includes a muffler to attenuate the noise associated with the flow of exhaust gas from the engine. Unfortunately, as explained further herein, mufflers necessarily impose a back pressure on the flow of the exhaust gas. Engine efficiency varies generally inversely with the level of back pressure in the exhaust system. Thus, higher back pressures reduce engine efficiency and fuel economy, while lower back pressures enable the engine to operate more efficiently.
Prior art mufflers having only a single straight-through tube, will provide low back pressure and therefore will have a minimal adverse effect on engine efficiency. Examples of these prior art mufflers are the "glasspacks" that are used by hot-rodders for optimum engine performance. A glasspack typically will include a single linear perforated or louvered tube disposed in a tubular outer shell and with a fiberglass noise insulation disposed between the perforated or louvered tube and the outer shell. Although prior art mufflers of this type may achieve a low back pressure, they are not effective in attenuating noise, and do not achieve the noise attenuation requirements for new automotive vehicles in the United States.
Exhaust mufflers on most new cars are very effective in attenuating noise, but create significant back pressure with a corresponding negative effect on engine performance and efficiency. A prior art muffler is illustrated in FIG. and is identified generally by the numeral 10. The muffler 10 comprises a plurality of separate tubes, 11-13 which are supported in a parallel array by transversely extending baffles 14 and 15. The baffles 14 and 15 typically are of oval or circular configuration corresponding to the selected cross-sectional size and shape for the muffler 10. Portions of the tubes 11-13 disposed between the baffles 14 and 15 may be perforated or louvered to permit a controlled expansion of exhaust gas from each tube 11-13, and to permit some communication therebetween. The tubes 11-13 and baffles 14 and 15 of the prior art muffler 10 are disposed within a tubular outer shell 16 of generally oval or circular cross-sectional configuration conforming to the shape of the baffles 14 and 15. End caps 17 and 18 ar mounted to the opposed ends of the outer shell 16 to substantially enclose the tubes 11-13. The end cap 17 is provided with an aperture to enable the exhaust pipe of the exhaust system to communicate with the tube 11. Similarly, the end cap 18 is provided with an aperture to enable the tube 13 to communicate with the tail pipe of an exhaust system. This typical prior art muffler 10 defines a total of three chambers 19, 20 and 21. With this prior art construction, exhaust gas from the engine will enter the tube 11. A controlled amount of expansion will occur in the perforated region of the tube 11 passing through the chamber 20. Most of the exhaust gas, however, will flow from the tube 11 and will abruptly expand into the chamber 21, then will undergo a 180.degree. change of direction to enter the tube 12. The well defined edges of tubes 11 and 12 create turbulence and back pressure on the exhaust gas flowing therebetween. Once again, some expansion will occur as the exhaust gas in the tube 12 passes through the chamber 20. However, most exhaust gas will flow through the tube 12 and into the chamber 19. The exhaust gas will expand abruptly again and will undergo another 180.degree. change of direction to enter the tube 13. The exhaust gas will then travel once again through the chamber 20 and toward the tail pipe connected to the tube 13. Turbulence and back pressure again will be created by the raw edges of the tubes 12 and 13. It will be appreciated that many more complex variations of this prior art muffler 10 exist, including mufflers having more than three pipes and more than two transverse baffles. Furthermore, the dimensions and locations of the components will vary in accordance with the needs of the system.
Although the prior art muffler 10 is very effective in attenuating noise, it suffers from several significant deficiencies. First, the abrupt expansion and the 180.degree. changes in direction which take place in the chambers 21 and 19 respectively create significant back pressure with corresponding negative effects on engine efficiency. It is estimated that this prior art muffler 10 will reduce engine efficiency by 10%-30%, with the exact percentage being dependent on various parameters of the system, including how well the muffler is designed. Attempts have been made to enhance efficiency by providing concave reflecting surfaces in the chambers in which such changes of direction take place. However, these attempts do not significantly offset the eddying motion of exhaust gas which is responsible for a large loss of flow energy and a high pressure drop for the total system. The typical prior art muffler 10 also is undesirable in that it requires a large number of separate parts that must be assembled in a labor intensive manufacturing process. Additionally, the prior art muffler 10 affords few options in designing the muffler to fit the available space on the vehicle. In this regard, the prior art muffler 10 is substantially limited to a uniform circular or oval cross-sectional shape with an inlet at one end and outlet at the opposed end. To conform with these shape limitations the exhaust pipe and tailpipe often must undergo long sweeping turns which add significantly to the length of these pipes with corresponding increases in both cost and weight.
Mufflers formed at least in part from stamped components have been available for many years. The typical prior art stamp formed muffler has included a pair of opposed internal plates that are stamped to define a circuitous perforated tube therebetween. A pair of external shells are stamped to define at least one chamber surrounding the perforated tube. These prior art stamp formed mufflers are well suited to automated manufacturing techniques and therefore offer some manufacturing efficiencies over the above-described an illustrated conventional prior art muffler. Examples of prior art stamp formed mufflers of this general type are shown in British Patent No. 632,013 was issued to White in 1949; British Patent No. 1,012,463 was issued to Woolgar on Dec. 8, 1965; Japanese published Patent Application No. 59-43456 which was published in 1984; and U.S. Pat. No. 4,132,286 was issued to Hasui et al on Jan. 2, 1979. These mufflers may eliminate a broad range of the noise associated with the flow of exhaust gases. However, most mufflers that rely entirely on perforated tubes and expansion chambers fail to attenuate at least one fairly narrow range of low frequency noise associated with the flow of exhaust gases. Consequently, prior art mufflers of this type have been employed on lawnmowers and chainsaws where noise attenuation is less critical and on some European sports cars where a low frequency residual noise is acceptable and/or desirable. Mufflers of this general type have not been accepted on new cars in the United States where more stringent noise control is required.
The prior art further includes mufflers having a circuitous array of nonperforated tubes and chambers arranged in series for the exhaust gas to flow through. Examples of this type of prior art muffler include U.S. Pat. No. 3,176,791 was issued to Betts et al. on Apr. 6, 1965 and U.S. Pat. No. 3,638,756 was issued to Thiele on Feb. 1, 1972. One muffler depicted in U.S. Pat. No. 3,638,756 shows a single flow tube communicating with an in-line expansion chamber. These mufflers also have not been commercially accepted on automotive vehicles.
Still other prior art mufflers include conventional tubular components disposed within a stamped outer shell. Mufflers of this general type are shown in U.K. Patent Application No. 21 120 318 and U.S. Pat. No. 4,109,751 which issued to Kabele on Aug. 29, 1978. These prior art mufflers may offer some manufacturing efficiencies, but generally suffer from the back pressure problems of the conventional prior art muffler depicted on FIG. 1.
The recent prior art includes several very significant advances in stamped muffler technology. In particular, U.S. Pat. No. 4,700,806 issued to Jon Harwood on Oct. 20, 1987 and is assigned to the assignee of the subject application. The muffler in U.S. Pat. No. 4,700,806 is uniquely constructed from stamped components to provide at least one tuning tube, at least one low frequency resonating chamber communicating with the tuning tube, and at least one expansion chamber communicating with at least one other tube in the muffler. This unique combination enables the muffler shown in U.S. Pat. No. 4,700,806 to achieve noise attenuation that is at least equal to the attenuation enabled by the conventional prior art muffler depicted in FIG. 1 above. Additionally, the muffler in U.S. Pat. No. 4,700,806 achieves the various manufacturing efficiencies available with stamped technology, and has been found to provide significantly lower back pressure levels than the conventional muffler as depicted in FIG. 1. The lower back pressure levels are at least partly attributable to the smoothly curved tubes stamped into the internal plates to effect changes of direction for the exhaust gas traveling through the muffler. Furthermore, the cross-sectional dimensions of the tubes can be selectively changed along the flow path to optimize both noise attenuation and back pressure. The disclosure of U.S. Pat. No. 4,700,806 is incorporated herein by reference.
The assignee of the subject application has made several other significant advances in stamped muffler technology. For example, U.S. Pat. No. 4,760,894 shows the use of the stamp formed technology to provide a muffler having angularly aligned inlets and outlets to achieve and efficient routing of pipes to and from the muffler. U.S. Pat. No. 4,821,840 and U.S. Pat. No. 4,909,348 both show the use of stamped muffler technology to efficiently nest the muffler into the available shape on the vehicle. U.S. Pat. No. 4,765,437 shows stamp formed mufflers having plural low frequency resonating chambers and an expansion chamber with only a single baffle crease being formed in each external shell of the muffler. U.S. Pat. No. 4,836,330 shows a stamp formed muffler with an expansion chamber, a plurality of low frequency resonating chambers, and with only a single tube crossing the baffle crease to avoid creating pockets that conceivably could accumulate corrosive materials. Pending U.S. patent application Ser. No. 471,288 also is assigned to the assignee of the subject invention and shows a muffler with a transverse tube aligned with the baffle crease of the external shells to minimize the amount of deformation in the baffle crease and to avoid creating pockets. The disclosures of the above-referenced patents and the application that are assigned to the assignee of the subject invention are incorporated herein by reference.
Despite the many advantages of the stamp formed mufflers developed by the assignee of the subject invention, there is still the desire to further improve exhaust system technology. For example, new car manufacturers are subject to increasing pressure to enhance fuel efficiency and engine performance. One approach to enhancing fuel efficiency is to reduce the back pressure provided by the exhaust system. Although the above-described stamped muffler technology reduces back pressure over the conventional prior art muffler, it is desired to provide even further reductions in back pressure.
Fuel efficiency also can be improved by reducing vehicular weight. A muffler that requires less metal necessarily would be lighter and therefore could contribute proportionally to fuel efficiency. Lightweight mufflers require less material and therefore may cost less. In this regard, the automotive industry is very competitive, and even small savings in cost can be significant Many of the above-described prior art stamp formed mufflers that are assigned to the assignee of the subject invention are stamped to include a baffle crease that is unitary with the external shell and that separates chambers of the muffler. The unitary baffle crease has been found to be an extremely effective and efficient means for forming a plurality of chambers. An entirely separate baffle, on the other hand, would require different stamping dies and a more complex assembly process. However, both unitary baffle creases and separate baffles may add to the total amount of metal required for the muffler, thereby adding to costs and weight. For these reasons, a muffler that eliminates both separate baffles and unitary baffle creases could be desirable in some situations.
It is known that desirable sound attenuation can be achieved by directing the tube of a muffler into a comparatively very large chamber or "expansion can" which permits substantial expansion of the exhaust gas. Attenuation at any selected frequency generally increases with the ratio of the chamber's cross-sectional area to the inlet tube's cross-sectional area. However, the limited available space on the underside of a vehicle generally has prevented the use of a very large in-line expansion chamber into which an incoming tube may communicate. Conversely, the use of a very small inlet tube would create significant back pressure on the prior art muffler with the above-described negative effect o engine performance. A general discussion of in-line expansion chambers is provided in NACA Report 1192 "Theoretical and Experimental Investigation of Mufflers with Comments on Engine--Exhaust Muffler Design" by Don D. Davis Jr. et al. The mufflers shown in NACA Report 1192 all have conventional tubes with well defined edges leading into the in-line expansion chamber, and thus create turbulence and back pressure as explained above. As noted above, U.S. Pat. No. 3,638,756 shows an in-line expansion chamber in a muffler formed entirely from stamped components. However, space limitations and back pressure requirement would severely limit the range of expansion ratios that could be achieved with the muffler of U.S. Pat. No. 3,638,756.
Still another version of a prior art muffler is shown in U.S. Pat. No. 4,809,812 which issued to Flugger on Mar. 7, 1989. The muffler shown in U.S. Pat. No. 4,809,812 is manufactured substantially from conventional tubes and/or baffles disposed in a tubular outer shell. A single inlet tube of the muffler shown in U.S. Pat. No. 4,809,812 is divided into two substantially identical and symmetrical flow tubes which are then directed back toward one another from opposed directions. The recombined flow tubes may then lead to a second pair of divided then recombined flow tubes or to a chamber. The theory of U.S. Pat. No. 4,809,812 is that the direction of the initially divided flows against one another will attenuate noise. In practice, however, the muffler of U.S. Pat. No. 4,809,812 has not performed well accoustically.
Mufflers with Venturi tubes have been experimented with in the past. A Venturi tube defines a tubular section with a localized restriction. By carefully selecting the cross-sectional area of the Venturi tube restriction with respect to the upstream and down-stream tube cross-sections and by carefully selecting the location of the Venturi and the shape of the tapers leading into and out of the Venturi it is believed that positive effects on back pressure and noise attenuation can be achieved. Venturi tubes have been difficult and costly to incorporate into the conventional prior art muffler as shown in FIG. 1. Furthermore, it has been difficult to design Venturi tubes in mufflers that will achieve the theoretical benefits.
In view of the above, it is an object of the subject invention to provide a muffler that enables substantial improvements in engine performance.
It is another object of the subject invention to provide a muffler that efficiently attenuates noise.
A further object of the subject invention is to provide a muffler having a low profile.
Still an additional object of the subject invention is to provide a muffler that utilizes less metal material.
Yet a further object of the subject invention is to provide a stamp formed muffler that avoids deep draws of metal material during the formation of the muffler.